1. Field of the Invention
This invention relates to light emitting element drive circuits. More particularly, the invention is directed to a light emitting element drive circuit that is low in power consumption and provides a light output of high quality. The invention is useful in optical communication systems
2. Description of the Related Art
Typically, an optical communication system includes an optical transmitter and an optical receiver. In the transmitter, a drive circuit applies a data-modulated drive current to a light emitting element, which optically transmits the data. It is desirable that this light emitting element drive circuit is consume little power and be able to handle high data rates such as 100 Mbps.
An LED (light emitting diode), which is a typical example of the light emitting element, has a relatively large capacitance. Therefore, when its drive signal charges from a "light" state to a "no light" state, a relatively long period of time elapses before the light emission ceases. To cope with this problem, the LED may be driven by a complementary drive system as disclosed by Japanese Patent Application 234568/1988 (unexamined, but published).
FIG. 2 (prior art) shows the typical arrangement of the conventional light emitting element drive circuit.
The conventional light emitting element drive circuit of FIG. 2 essentially comprises a pair of NPN transistors Q.sub.1 and Q.sub.1, the emitters of which are commonly connected to a current source I, and a PNP transistor Q.sub.3, the base of which is connected through a capacitor C.sub.1 to the base of the transistor Q.sub.1. The collector of the transistor Q.sub.1 is grounded through a light emitting diode LED. The collector of the transistor Q.sub.3 is connected to the collector of the transistor Q.sub.1. The emitter of the transistor Q.sub.3 and the collector of the transistor Q.sub.4 are grounded. An input signal SD and its inverted signal SD are applied to the bases of the transistors Q.sub.1 and Q.sub.1, respectively. A bias is applied through a resistor R.sub.1 to the base of the transistor Q.sub.3.
The conventional drive circuit operates as follows: When the input signal SD rises, transistor Q.sub.1 conducts. Current flows through the light emitting diode LED and the optical signal output of the diode rises. When the input signal SD falls, the transistor Q.sub.1 is rendered non-conductive so that the application of current to the light emitting diode LED is interrupted. At the same time, the inverted signal SD rises to render the transistor Q.sub.2 conductive so that current flows through the conductive transistor Q.sub.2.
Input signal SD is applied to the base of the transistor Q.sub.3 through a differentiating circuit including resistor R.sub.1 and a capacitor C.sub.1. When the input signal SD falls, the transistor Q.sub.3 is rendered conductive so that the light emitting diode LED is discharged through the transistor Q.sub.3, thus minimizing the amount of time that elapses before the output optical signal attributed to the capacitance of the light emitting diode LED ceases.
The circuit that outputs the signal SD which is applied to the differentiating circuit made up of the resistor R.sub.1 and the capacitor C.sub.1 must be low in impedance. Therefore, the current consumption is unavoidably increased.
Because the base of the transistor Q.sub.1 which drives the light emitting diode LED directly is coupled to the base of the transistor Q.sub.3, the waveform of the output optical signal is likely to be deteriorated by so called "back gating".